This invention relates generally to copying or printing apparatus, and more particularly, it relates to the dispensing of particulate toner material in a two-component development system.
In an electrophotographic copying and/or printing machine, a charge retentive surface such as a photoconductive member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to light patterns representative of an image being reproduced. Exposure of the charged photoconductive member selectively to light dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the desired information. After the electrostatic latent image is recorded on the photoconductive member, it is developed by bringing a developer material into contact therewith. This forms a powder image on the photoconductive member which is subsequently transferred to a copy sheet. The powder images on the sheet are heated to permanently affix the marking particles thereto in image configuration.
In the foregoing type of printing machine, a development housing is employed to deposit toner material onto the electrostatic latent image recorded on the photoconductive surface. The latent image may be developed with a two component developer. In a two-component developer, fine toner particles adhere triboelectrically to coarser carrier granules. Typically, the toner particles are made from a thermoplastic material while the carrier granules are made from a ferromagnetic material.
All two component development housings require continuous replenishment of toner to a developer sump as prints are made. Toner addition is the dominant control mechanism for most xerographic machines. Usually a print quality sensor like an IRD looking at developed patches on a photoreceptor, or a toner concentration sensor like a permeability or conductivity control device, are used to turn a toner dispenser on or off. Toner dispenser design is critical to the maintenance of xerographic print quality.
Toner addition is often achieved by attempting to uniformly disperse the toner across the width of the development housing. This is often done by devices like a foam roll, a brush and screen, a thumper or shaker design. In practice, these usually do not produce uniform dispensing of the toner. Often it is dispersed in clumps across the width of the housing. Toner addition is also often achieved by adding it at one end of an auger that also captures some quantity of circulating sump developer. All these dispensers attempt to keep toner concentration under control by adding toner in bursts. After toner is dispensed to the developer in this manner additional hardware is always required to uniformly blend the added toner. This is often achieved with complex passive cross mixing baffles, coned fins on paddle wheels or multiple auger arrangements.